Molten carbonate fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, an electrolyte tile making contact with both the electrodes, and a cell housing to physically retain the cell components and to provide contacts between the electrodes and the reactant gases. Under fuel cell operating conditions, in the range of about 500.degree. C. to about 700.degree. C., the entire electrolyte tile consisting of the carbonate, and the inert support material, form a paste and thus, the electrolyte diaphragms of this type are known as paste electrolytes. The electrolyte is in direct contact with the electrodes where the three phase reactions (gas-electrolyte-electrode) take place. Hydrogen is consumed in the anode area producing water, carbon dioxide and electrons. The electrons flow to the cathode through an external circuit producing the desired current flow. At the anode, there must be ready entry for the reactant gas, ready exit for the chemical reaction products and ready exit for the product electrons. To maintain a high level of stable performance, both electrolyte and electrode design and properties must be optimize and stabilized at the gas-electrolyte-electrode interface.
Porous anodes of cobalt, copper, or nickel have been previously used in molten carbonate fuel cells. These anodes typically require stabilizing agents to maintain porosity and surface area during fuel cell operation. The stabilizing agents are usually added in about 1-10 weight percent, based upon the metal. The stabilizing particles are dispersed on the base metal surface, prohibiting the structure from sintering at molten carbonate fuel cell temperatures of 500.degree. C. to 700.degree. C.
The molten carbonate fuel cells have typically used nickel, cobalt, and copper-based anode structures. These anodes tend to be dimensionally unstable, losing thickness by creep deformation within the fuel cell stack.
The porous anodes described have problems in that they are prone to creep deformation which occurs as a result of a holding force applied to keep the components in good contact. Creep of electrodes occurs by a combination of at least three different creep mechanisms: particle rearrangement, sintering, and dislocation movement. The surface dispersion of stabilizing particles used in prior art methods does not inhibit creep by dislocation movement. The creep of these anodes under the loaded conditions of a fuel cell stack is not acceptable.
Various methods have been used to attempt to inhibit creep deformation in the anode structures. One method has been to internally oxidize the alloying metal used in the base metal-alloying metal composition typically used to form the porous anode structures.
For example, U.S. Pat. No. 4,315,777 to Nadkarni discloses the internal oxidation of a powder that is precompacted. The powder is a blend of an alloy powder and an oxidant base metal. Upon heat treatment, the alloying metal becomes internally oxidized by oxygen supplied by the metal oxide contained in the oxide powder. Thus, the structure of Nadkarni contains oxidant particles which are, in effect, additives and which provide oxygen for internal oxidation of the alloying metal. This method of dispersion strengthening utilizing an oxidant additive is disadvantageous since the properties of the alloy are compromised by side reactions with the additive which detracts from the performance of the dispersion strengthened alloy. Moreover, the method disclosed in Nadkarni produces a solid, highly dense end product which is not suitable for use as an electrode, which requires high porosity.
U.S. Pat. No. 4,594,217 to Samal discloses a process of forming highly dense bodies from oxide dispersion strengthened powders by cold rolling to a sheet having a density of at least 90% of the theoretical value prior to subsequent heat treatments. Samal uses a metal alloy as a starting material which is already dispersion strengthened and, thus, is not directed to a novel method of dispersion strengthening. Samal utilizes high pressure to bond the particles of the alloy composition together, creating highly dense bodies that are unsuitable for use as electrodes, which require high porosity.
U.S. Pat. No. 4,714,586 to Swarr discloses a method for forming dimensionally stable nickel-chromium anodes by internally oxidizing the alloying metal at high water vapor pressures. The oxygen used to internally oxidize the alloying metal is obtained externally from a gas atmosphere. Swarr is limited to the formation of nickel-chromium anodes.
U.S. Pat. No. 3,525,609 to Roberts discloses a dispersion strengthened copper alloy, which is alloyed with silver, cadmium, or zinc and strengthened by internally oxidized aluminum. The dispersion strengthened alloy is formed by internally oxidizing the alloy. The alloy is surface oxidized at 300.degree. C. and the internal alloying metal is then oxidized by heat treatment in a subsequent step. The alloy of Roberts forms highly dense bodies by the use of high pressure compaction techniques. These bodies are not suitable for use as porous anode structures as they are too dense.
U.S. Pat. No. 3,578,443 to Grant et al. discloses a method for producing an oxide dispersion strengthened alloy by surface oxidizing copper aluminum powder by intensive communition of the powder with an alcohol suspension. The surface oxidized powder is then heat treated in an air tight tube at 750.degree. C. to internally oxidize the entire powder. The communition can deform the powder particles to undesirable shapes, since they are unsuitable for anode microstructure design. The alloy is sintered in a later step. This process produces a highly dense body, unsuitable for use as a porous anode structure.
Thus, it is an object of the present invention to provide a method of preparing a porous anode structure which is dimensionally stable for molten carbonate fuel cell use.
It is a further object of the present invention to provide a method of preparing an anode of nickel and titanium that remains structurally stable with a minimum of creep deformation when in extended use under fuel cell stack conditions.
It is another object of the present invention to provide a method of preparing an anode of nickel and aluminum that remains structurally stable with a minimum of creep deformation when in extended use under fuel cell stack conditions.
It is still another object of the present invention to provide a method of preparing a porous anode structure that has improved creep resistance by internally oxidizing the alloying metal of the base metal-alloying metal composition which forms the alloying powder of the present invention by controlling the oxidation potential of the atmosphere and without using oxidant additives to the alloying powder.
It is a further object of the present invention to provide a method of preparing an oxide dispersion strengthened porous anode by surface oxidizing the alloying metal and base metal of the alloy powder, and subsequently simultaneously internally oxidizing the alloying metal and sintering, in one step, to form the anode structure.